In construction, modular surface systems are commonly used to reduce build cost and construction time. Modular surface systems typically allow for the rapid construction of floors, walls and ceilings, albeit often at the compromise of reduced aesthetic appearance. A prime example of such a modular surface system is a suspended ceiling, which can be found in most professional environments, such as for example office spaces. A suspended ceiling typically comprises a metal or plastic grid defining rectangular or square recesses, which are filled with panels or tiles to form a continuous ceiling.
In such modular systems, e.g. a suspended ceiling, lighting may be integrated into the system, typically by replacing one or more tiles with a lighting unit such as a luminaire. Most suspended ceilings comprise luminaires in which a number of fluorescent light tubes are present. For a number of reasons, such luminaires are not ideal. Firstly, such luminaires are considered aesthetically displeasing, i.e. obtrusive. Secondly, in order to improve light efficiency from such luminaires, they usually contain a reflector, which commonly has a parabolic shape. This however can cause glare for an occupant of the office space if the reflected light exits the luminaire under shallow angles to the plane of the modular system. Moreover, in applications where it is necessary to suppress noise levels, it may be required for the panels or tiles of the modular surface system to absorb sound, for which the aforementioned luminaires are unsuitable.
U.S. Pat. No. 6,367,581 B1 discloses a sound absorbing light fixture for a ceiling system. The light fixture comprises an air-filled cavity comprising a light source such as an incandescent light bulb in between a micro-perforated film acting as a porous light diffuser and a sound absorbing material. The air-filled cavity forms a standing wave sound field inside the cavity, in which the sound absorption coefficient can be optimized by improving the acoustic performance of the porous diffuser, the air-filled cavity and the sound absorbing material. Such light fixtures are however rather bulky.
It is known per se that small form factor lighting panels for modular surface systems can be achieved using solid state lighting elements such as light emitting diodes (LEDs). However, such solid state lighting elements are rather expensive compared to incandescent or fluorescent light sources, such that there is a desire to provide lighting panels comprising a small number of solid state lighting elements. Also, the heat produced by the solid state lighting elements limits the number of solid state lighting elements that can be applied per unit area of the lighting panel.
An example of a lighting panel including solid state lighting elements is disclosed in US 2004/0070967 A1. The acoustic light-emitting module includes a backing panel, a cover and a rigid spacing member extending between the backing panel and the cover, with a ring of solid state lighting elements such as LEDs placed inside the module. A problem with this acoustic light-emitting module is that the light output is inhomogeneous, which is aesthetically displeasing. Also, this solution requires a relatively large number of solid state elements, which is costly.
Another solution is to place solid state lighting elements at the edges of the lighting panels and use optical mirrors to produce a homogeneous light output. Such additional optics significantly increases the cost of the lighting panels. A solution avoiding such optical mirrors is disclosed in U.S. Pat. No. 3,752,974 in which uniform illumination is provided from an edge lighted optical medium of a given thickness by diffusing a surface portion normally providing internal reflection of the light radiated into the edge such that at least some of the light rays striking the diffused area are reflected through the opposite surface of the medium and some are refracted through the one surface. A cleared area about the source of light free of the diffusion is provided and treated by application of coatings on opposite surface portions of the medium defining the cleared area. These coatings have an index of refraction less than that of the medium such that light is channeled by internal reflection between the surfaces of the medium and inhibited from escaping from the medium in the vicinity of the cleared area. This cleared area has a radius of preferably three to four times the thickness of the optical medium. The resulting light emanation from the surface of the medium is substantially uniform beyond the cleared area. This solution however still is relatively complex, i.e. costly to manufacture.